Gas turbine engines, such as those used to power modern commercial aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. In aircraft engine applications, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor. An exemplary combustor features an annular combustion chamber defined between a radially inward liner and radially outward shell extending aft from a forward bulkhead. The radially inward liner forms a heat shield. The radially outward shell extends circumferentially about and is radially spaced from the inward liner. Arrays of circumferentially distributed combustion air holes penetrate the outward shell and the inward liner at multiple axial locations to admit combustion air into the combustion chamber along the length of the combustion chamber. A plurality of circumferentially distributed fuel injectors and associated swirlers or air passages are mounted in the forward bulkhead. The fuel injectors project into the forward end of the combustion chamber to supply the fuel. The swirlers impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead to provide rapid mixing of the fuel and inlet air. Commonly assigned U.S. Pat. Nos. 6,606,861 and 6,810,673, the entire disclosures of which are hereby incorporated herein by reference as if set forth herein, disclose exemplary prior art annular combustors for gas turbine engines.
Combustion of the hydrocarbon fuel in air inevitably produces oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. Accordingly, engine manufacturers strive to minimize NOx emissions.
One combustion strategy for minimizing NOx emissions from gas turbine engines is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL combustion strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a fuel-rich combustion zone at the forward end of the combustor, a quench or dilution zone that involves the conversion of rich combustion to lean combustion, and a lean combustion zone axially aft of the quench or dilution zone. Thus, the combustion process in a combustor configured for RQL combustion has two governing states of combustion: a first state in the forward portion of the combustor that is stoichiometrically fuel-rich and that is rapidly converted in a downstream second state that is stoichiometrically fuel-lean.
During engine operation with RQL combustion, a portion of the pressurized air discharged from the compressor is directed through a diffuser to enter the combustion chamber through the inlet air swirlers to support rich-burn combustion. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the front portion of the combustor. The resulting stoichiometrically rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture inhibits NOx formation in the rich burn zone by suppressing the combustion flame temperature. It also resists blowout of the combustion flame during certain operating conditions or any abrupt transients to engine power and promotes good ignition of the combustor.
The fuel rich combustion products generated in the first zone of combustion propagate where the combustion process continues. Pressurized air from the compressor enters the combustion chamber radially through combustion air holes. The air mixes with the combustion products from the first zone to support further combustion and release additional energy from the fuel. The air also progressively deriches the fuel rich combustion products as they flow axially through and mix with the air introduced in the quench region. Initially, with the air addition, the fuel-air ratio of the combustion products becomes less fuel rich approaching a stoichiometric composition, causing an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, significant quantities of NOx can be produced during the initial quench process where the combustion is rich. As quenching continues, the fuel-air ratio of the combustion products rapidly convert through the stoichiometric state to become fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate appreciable quantities of NOx.
Finally, the deriched combustion products after quench flow axially into the downstream of the combustor where the combustion process concludes as lean-burn combustion. Additional jets of compressor discharge air may be admitted radially into the lean burn zone. The additional air supports ongoing combustion to complete combustion of the fuel and to reduce the peak temperature, as well as regulate the spatial temperature profile of the combustion products prior to entering the turbine. Regulation of the peak temperature and temperature profile protects the turbine from exposure to excessive temperatures and excessive temperature gradients.
Another combustion strategy for minimizing NOx emissions from gas turbine engines is referred to as lean direct injection (LDI) combustion. The LDI combustion strategy also recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric. In LDI combustion, more than the stoichiometric amount of air required for combustion of the fuel is injected into the forward region of the combustion chamber and rapidly mixed with the inlet air to combust via a fuel-lean, as opposed to fuel-rich, process. A combustor configured for LDI combustion includes two serially arranged combustion zones: an overall fuel-lean combustion zone at the forward end of the combustor in which fuel and air are initially mixed followed by additional lean-burn combustion supported by dilution or cooling air addition in the axially aft portion of the combustor. The combustion process in a combustor configured for LDI combustion, by design intent, exists in one bulk governing state in which combustion is exclusively and stoichiometrically lean. Clearly, local conditions may not be lean given that mixing of the fuel and air require some finite time and spatial volume via mixing to achieve this state.
During engine operation with LDI combustion, a greater majority of the pressurized air discharged from the compressor is directed to the front end of the combustor through the inlet air swirlers, passages or a mixing chamber. The amount of air introduced as inlet air into the combustor is stoichiometrically excessive relative to the fuel injected concurrently through the front end. The resulting stoichiometrically fuel-lean fuel-air mixture is ignited and substantially combusted in the fore portion of the combustion chamber. The substantial excess of air in this zone inhibits NOx formation by suppressing the combustion flame temperature. The combustion products from this zone flow downstream with further mixing, potentially into a dilution zone, generally axially aft of the initial fuel lean burn zone. In the case of dilution, additional compressor discharge air is admitted radially into the combustor. The additional air contributes further to the combustion process and dilutes the combustion products thereby reducing the peak temperature, as well as regulating the spatial temperature profile of the combustion products prior to entering the turbine. Regulation of the peak temperature and temperature profile protects the turbine from exposure to excessive temperatures and excessive temperature gradients. Cooling air introduction into this second step can be supplemental to or in lieu of the dilution.
Most of the NOx emissions generated during combustion of a fuel in pressurized air in a gas turbine engine, whether by RQL combustion or LDI combustion, originates in high-temperature zones of localized, near-stoichiometric combustion conditions. These conditions occur despite the overall fuel lean nature of a LDI combustion chamber or the fuel-rich nature of the forward portion of a RQL combustion chamber. Thus, it is important to limit the time available for NOx formation. However, if combustion residence time, conventionally calculated by dividing the volume of the combustion chamber by the volumetric flow of gases through the combustion chamber, is to short or managed improperly, combustion of the fuel will be incomplete resulting in loss of power or performance trades, continued combustion in the turbine resulting in hot spots that occur in and damage the turbine, and possible increases in emissions, including carbon monoxide and hydrocarbons associated with combustion inefficiencies or complex physics.